CROSS-REFERENCE TO RELATED APPLICATION
Priority is claimed herein with respect to Application No. 100 56 232.9 filed in Germany on Nov. 13, 2000, the disclosure of which is incorporated herein by reference.
FIELD OF THE INVENTION
The invention relates to an optoelectronic device for detecting marks that are provided with defined contrast patterns, having a transmitter emitting transmitted light beams, a transmission lens downstream of the transmitter, a receiver that receives received light beams, a receiving lens upstream of the receiver, and an evaluation unit. The transmitted light beams are guided via the marks where they are reflected and then guided as received light beams to the receiver. Reception signals generated at the output of the receiver are evaluated in the evaluation unit in order to detect the marks.
BACKGROUND OF THE INVENTION
One such device is known from German Patent DE 43 37 718 C1, which is used to detect marks that have contrast patterns, and in particular to detect bar codes.
The transmitted light beams emitted by a transmitter are guided across the marks. The received light beams reflected from the marks have an amplitude modulation corresponding to the contrast pattern of the applicable mark.
The reception signals present at the output of the receiver because of the received light beams are evaluated in an evaluation unit, which has an n-bit analog/digital converter and an arrangement of digital filters. The analog/digital converter, which preferably has a word width of 8 bits, converts the analog reception signal into a digitized reception signal. This digitized reception signal is delivered to the arrangement of digital filters. The coefficients of the digital filters are selected such that the digital filtration compensates for signal distortions in the reception signal that are caused by component tolerances of the components used in the optoelectronic device.
The detection sensitivity of the optoelectronic device can be enhanced as a result.
From German Patent Disclosure DE 198 42 352, a further optoelectronic device for detecting marks, especially bar codes, is known. In this device, a transmitter is used that emits transmitted light beams in the blue wavelength range.
In comparison to optoelectronic devices that use transmitters that emit transmitted light beams in the red or infrared wavelength range, in this device, with increasing distance, a slower widening of the beam diameter of the transmitted light beams takes place. As a consequence, the transmitted light beams, even at relatively great distances from the device, still have such a small beam diameter that secure detection of the marks is assured.
Thus by the use of such transmitters, a great depth of field, within which marks are detectable, is obtained.
SUMMARY OF THE INVENTION
An object of the invention is to furnish an optoelectronic device of the type defined at the outset which with the smallest possible size has the highest possible detection sensitivity.
The above and other objects are achieved according to the invention by the provision of an optoelectronic device for detecting marks having defined contrast patterns, comprising: a transmitter for emitting transmitted light beams having a wavelength λ in a range of 350 nm ≦λ≦450 nm; a transmission lens downstream of the transmitter; a receiver for receiving received light beams and generating reception signals corresponding to the received light beams; a receiving lens located upstream of the receiver and having an area AC of less than or equal to 5 cm2; means for guiding the transmitted light beams at the marks and for guiding reflected light beams from the marks as received light beams to the receiver; and an evaluation unit coupled to an output of the receiver for evaluating the reception signals.
With an optoelectronic device embodied in this way, the advantages of high detection sensitivity and small structural size can be combined.
The invention is based on the recognition that the limits of detection of optoelectronic devices used industrially to detect marks, especially at great distances of the marks from the device, are determined by the speckle noise.
The speckle noise is caused by the fact that the marks themselves, or the substrates onto which the marks are printed, have a certain surface roughness.
The transmitted light beams striking such a surface, and especially a paper surface, are diffusely reflected from it.
Because of the irregular, rough surface structure, optical interference occurs in the transmitted light beams that are reflected from the marks and guided to the receiver in the form of received light beams. Thus in the reception plane of the receiver, so-called speckles appear in the form of spot-like interference patterns.
When a mark is scanned by the optoelectronic device, the transmitted light beams are guided across the mark. Since the roughness structure of the surface on which the mark is applied is not distributed homogeneously but rather statistically, the speckled pattern on the receiver varies during the scanning. This variation over time of the speckled pattern on the receiver causes the speckle noise.
The signal-to-noise ratio SNR that is obtained because of the speckle noise is proportional to the quotient of the area of the receiving lens AC and the mean area <A> of the speckle:
The mean area <A> of the speckle in turn depends on the distance d of the mark from the wavelength <R> of the roughness of the surface, in accordance with the following relationship:
<A>˜d 2·π2 /<R> 2
The result is the following dependency for the signal-to-noise ratio:
SNR˜A C /<A>˜A C <R> 2/(d˜λ)2
According to the invention, the speckle noise is minimized in the optoelectronic device by the provision that instead of transmitters that conventionally emit in the red or infrared wavelength range, a transmitter is used that emits transmitted light beams in the ultraviolet and/or blue wavelength range. According to the invention, a transmitter is used that emits transmitted light beams having a wavelength λ in the range of 350 nm ≦λ≦450 nm.
The transmitters typically used in industrial applications emit at a wavelength of about 660 nm. If that kind of transmitter is replaced by a transmitter that emits transmitted light beams at about 400 nm, then the signal-to-noise ratio is improved by a factor of approximately 2.7.
According to the invention, the use of a transmitter that emits in the ultraviolet or blue wavelength range is combined with a receiving lens whose area is less than or equal to 5 cm2.
The use of such a small receiving lens makes a considerable reduction in the structural size of the optoelectronic device of the invention possible, compared to known devices of this type.
This is due to the fact that the size of the receiving lens essentially determines the entire structural size of the optoelectronic device. This is especially true whenever the optoelectronic device has a deflection unit, which in particular has a rotating polygonal mirror wheel.
By means of the polygonal mirror wheel, the transmitted light beams are periodically guided to inside a scanning range. The received light beams reflected from the marks are guided via the polygonal mirror wheel to the receiver.
In order for the received light beams to be guided all the way from the polygonal mirror wheel to the receiver, the sizes of the mirror faces are adapted to the area of the receiving lens across which the received light beams are guided to the receiver.
Thus the smaller the receiving lens can possibly be made, then the smaller the polygonal mirror wheel can be. Because of its relatively large volume, the size of the polygonal mirror wheel definitively determines the size of the entire device, so that by reducing the area of the receiving lens, the entire structural size of the optoelectronic device can be reduced.
The smaller area of the receiving lens is compensated for, or overcompensated for, by the use according to the invention of the transmitter that emits in the ultraviolet and/or blue wavelength range, so that compared to conventional optoelectronic devices, which use transmitters that emit in the red or infrared wavelength range, a considerable reduction in structural size is obtained, for the same or even improved detection sensitivity.
The invention will be described below in conjunction with the drawing.